Electrostatic system monitor and method therefor

ABSTRACT

An electrostatic system monitor and method for monitoring the operation of static control equipment such as conductive workbench and floor mats, conductive wrist straps, and the integrity of ground connections and soldering irons and to activate indicators and alarms when the static control equipment deteriorates or malfunctions by having resistances outside of predetermined values and ranges of resistance. The electrostatic system monitor also provides for the detection of unsafe voltage potentials on soldering irons, and for a surface resistance probe for measuring the surface conductivity of conductive surfaces of the static control equipment.

DISCUSSION OF THE PRIOR ART

1. Field of the Invention

This invention relates to the field of control of electrostaticdischarge, and, more particularly, to the monitoring of static controlsystems to assure substantial elimination of electrostatic discharge inthe manufacturing environment of electronic solid state circuitry.

2. Background of the Art

It is well known that static electricity can cause substantial damage tosolid state electronic circuitry. Dry climates generally produce higherstatic electricity charges than humid climates and, for example, aperson walking across a nylon carpet when humidity is 20 percent or lesscan easily buildup a static electric charge of 30,000 volts. Generallyit takes approximately 5,000 volts to generate a spark but only aslittle as 100 to 500 volts of static electricity to damage sensitiveintegrated circuits. Even in a high humidity environment, a personwalking across a tiled floor may buildup a static voltage of 500 to2,000 volts. While such a charge is not high enough to cause noticeableelectric shocks, it is certainly more than enough to damage sensitiveintegrated circuits.

The present invention provides a systems approach to the monitoring ofconventional static control equipment and the substantial elimination ofelectrostatic discharge damage to sensitive solid state circuitry,semi-conductors, and integrated circuits. Because of the recent, rapidreplacement by metal oxide semi-conductor (MOS) integrated circuits ofthe older, and less static damage susceptible rugged bi-polar TTLintegrated circuits, the problem of electrostatic discharge is now evengreater. This is due, in part, to the gate structure of MOS devices andalso, in part, to the higher density and smaller physical size of MOSintegrated circuitry.

The types of damage caused by electrostatic discharge to integratedcircuits constitutes two general areas. The first area of damage occurswhen the static discharge physically causes a part or parts of anintegrated circuit to become inoperative which will be discovered inmanufacturing during quality control testing. The occurrence of thistype of damage is costly because it occurs typically after the productis assembled. The second area of damage is even more costly. It has beendetermined that approximately eighty percent of all electrostaticdischarge damage is latent; that is, a part or parts of the integratedcircuitry initially gets overstressed due to electrostatic discharge,but would still meet the minimum quality control and test requirements.Such overstressed integrated circuits will, however, always continue todegrade with time and will fail within a relatively short period oftime. The costs involved with this type of failure, therefore, areenormous, because it relates to unhappy clients, return of merchandise,increase field service costs and expenses, and the like. It is crucial,therefore, to properly equip work stations with static control equipmentand then to monitor the proper operation of this equipment which isprovided by the present invention.

To combat such electrostatic discharge damage, work station operatorsassembling electronic products containing such circuitry are physicallygrounded by means of conventionally available wrist straps wired toground to remove the static buildup of electricity within a person'sbody. Such operators can buildup high electrostatic voltages often inthe order of 500 to 30,000 volts. A conductive wrist strap connected tothe operator's wrist and then connected to ground minimizeselectrostatic discharge damage caused by the work station operator. Tofurther minimize electrostatic discharge damage, it is common practiceto use conductive bench and floor mats in the operator's work stationarea which are also connected to ground. It was common for work stationsurfaces and storage containers to be made from plastic materials.However, the use of such materials only enhanced the electrostaticdischarge damage and, therefore, conductive plastic materials for benchtops, floor mats, and storage containers were developed. Such conductivebench tops and floor mats were grounded to safely discharge any staticelectricity. The use of such mats and storage containers further insurethat the generation of electrostatic discharge will be essentiallyeliminated. The common ground connection for the entire static controlsystem, should preferably be made to a nearby water pipe or any othersuitable earth ground.

Another source of damage in the assembly of such electronic products isthe use of an ungrounded or defective soldering iron by the work stationoperator. A typical ungrounded soldering iron may generate 200 volts ACpeak at the tip of the soldering iron and if the iron is SCR-controlled,more voltage can be generated. Soldering irons are now designed to havethe tip connected to ground to prevent the generation of such ACvoltages.

In the non-assembly environment, various electronic products are nowbeing packaged in conductive plastic packaging material and operators ofthese electronic products often use static control sprays to reduce thebuildup of static discharge. The above described static controltechniques are now widely used by the electronics industry and arerecognized to result in the dramatic reduction in losses fromelectrostatic discharge damage. MOS devices now constitute more than 80%of total integrated circuit usage, a fact which also adds to theseverity of the problem.

While the above measures have been an important factor in reducingelectrostatic discharge damage, there is no general way of verifying thecontinued integrity of these static control measures. For example, wriststraps, ground wires and soldering irons may become deteriorated and maydevelop open ground connections due to the constant handling and flexingby the work station operator. When such failures occur, the operator hasno ready means of detecting such failure in the equipment. In a typicalmanufacturing environment containing hundreds of work stations, theprobability of one or more of the work stations having such defectiveequipment is high. When such defective or inoperative static controlequipment is allowed to continue in service, without monitoring andrepair, substantial damage to equipment and monetary loss can occur.

It is the goal of the present invention to continuously monitor bothsystem earth ground and system electrical ground, monitor the groundconnection and the continuity of the wrist strap and the conductivebench and floor mats, and detect any damaging voltage on the tip of thesoldering iron. Furthermore, a convenient conductivity test capable ofmeasuring the surface area resistance of the conductive mats and othertypes of conductive materials is further provided. The cost of thestatic system monitor of the present invention to provide thiscontinuous monitoring is minimal when compared to the savings caused bypreventing damage to the electronic product. Under the teachings of thepresent invention, any failure or abnormal condition will activate bothan audible alarm and clearly marked visual warning lights.

SUMMARY OF THE INVENTION

The present invention concerns an electrostatic system monitor andmethod for monitoring the operation of various static control equipmentin order to assure the substantial elimination of static electricity.Such static control equipment conventionally available includesconductive workbench mats, conductive floor mats, grounded wrist strapsover the wrists of users, and grounded soldering irons. The workbenchmat, floor mat, and wrist strap are conventionally connected to an earthground connection such as that of a water pipe.

The electrostatic system monitor of the present invention includes acontinuous continuity test circuit for monitoring the low impedancecontinuity between electrical ground and earth ground. In the event thatthe impedance increases above a predetermined resistance value, thecircuit issues a loss of continuity signal to an indicator and an alarm.The monitor of the present invention also includes a circuit responsiveto the periodic manual testing of the conductive wrist strap todetermine the resistance of the conductive wrist strap. In the event theresistance is either above or below a predetermined resistance range, apredetermined indicator arrangement is activated. Likewise, the presentinvention is responsive to the periodic manual testing of the tip of thesoldering iron to detect whether the induced AC voltage on the tip ofthe soldering iron exceeds a predetermined AC voltage and, if so, asuitable indicator and alarm is activated. The present invention alsoincludes a surface resistance probe for detecting the surface resistanceof the conductive workbench mat and floor mat or any other conductivesurface. When the surface resistance of the conductive surface is eithergreater than or less than a predetermined adequate conductivity, aprearranged pattern of indicators and alarms are also activated.

In this fashion, the electrostatic system monitor of the presentinvention provides a simple and effective monitor of conventionallyavailable static control equipment to ascertain that the equipment isfunctioning properly. A central monitor is provided for supervising anumber of electrostatic system monitors at workstations in amanufacturing environment.

DESCRIPTION OF THE DRAWINGS

FIG. 1 sets forth a work station utilizing the electrostatic systemmonitor of the present invention,

FIG. 2 sets forth a block diagram arrangement showing a plurality ofelectrostatic system monitors interconnected with a single centralmonitor station,

FIG. 3 sets forth the electronic circuitry for the electrostatic systemmonitor of the present invention,

FIG. 4 is an exploded perspective view of the surface resistance probeof the present invention, and

FIG. 5 is a side planar view illustration showing the application of thesurface resistance probe of the present invention to an uneven surface.

GENERAL DESCRIPTION

In the following, the various functions of the electrostatic systemmonitor of the present invention will be discussed and includes: aground continuity test, a wrist strap test, a conductivity test, and asoldering iron test.

1. Ground Continuity Test

In FIG. 1 is set forth the electrostatic system monitor 10 of thepresent invention as set up on a manufacturing static control workbendlocation. The electrostatic system monitor 10 is located on a conductiveworkbench 15 having a backing 30 containing electrical conduit andelectrical outlets 40. A conductive bench mat 20 and a conductive floormat 25 are provided. Conductive bench mat 20 is grounded over wire 17and conductive floor mat 25 is grounded over wire 27. The electrostaticsystem monitor 10 of the present invention obtains its AC power andelectrical ground over cord 50 which is plugged into one of the outlets40 on the utility backing 30. The electrostatic system monitor 10 of thepresent invention also receives earth ground over cable 60 which isconnected to a conventional clamp 70 on water pipe 80. Cable 60 ispreferably fourteen to eighteen gauge wire which is run to the nearestwater pipe 80. The remaining end of cable 60 is connected to an earthground terminal 90 by means of an opposing electrical mating connector100. As noted, water pipe ground wire 60 also connects to wires 17 and27 for grounding of conductive mats 20 and 25. Both wires 17 and 27 mustbe grounded to the common ground from water pipe 80. This may beaccomplished by splicing wire 17 and 27 to wire 60, as shown, which isthen connected to water pipe 80, or by connecting wires 17 and 27 to the"binding post" terminal 90, which can accept several wires.

With the electrostatic system monitor of the present invention 10receiving power over cable 50, the system will continuously monitor forlow impedance continuity between the earth ground provided by the waterpipe 80 and the electrical ground provided by the electrical outlet 40.If the resistance between the two grounds is less than a predeterminedvalue such as approximately 100 ohms, a "GROUND OK" light 110 is turnedon. If the resistance exceeds the predetermined value of low impedance,light 110 goes out and audible alarm 130 sounds.

2. Wrist Strap Test

In another aspect of the present invention, the human operator 11located at the work station and who might be charged with electrostaticelectricity, wears a conductive wrist strap 120, strapped onto the wristand interconnected over cable 131 to the electrostatic system monitor 10of the present invention. Cable 131 has a connector 140 which mates withconnector 150 on monitor 10. Interconnecting cable 131 provides earthground to the wrist of the person 11 and effectively discharges anyelectrostatic electricity contained on the human operator 11.

Furthermore, the present invention will determine whether the wriststrap 120 has proper resistance by one of two approaches. First, theoperator touches the wrist strap 120 which is conductive by makingcontact with the wrist strap test terminal 160. The "Strap/Conductivity"test indicator 170 will become activated if the wrist strap 120 has asafe resistance in a predetermined strap resistance range of more thanapproximately 200 kilohms, but less than approximately 2.5 Megohms. Inother words, the safe strap resistance operating range of the preferredembodiment is between 200 Kilohms and 2.5 Megohms. However, if the wriststrap 120 is open or has deteriorated to a resistance of more than 3.0Megohms, then the visual indicator 170 will remain off. On the otherhand, if the wrist strap 120 is shorted or has deteriorated to less than150 Kilohms, the audible alarm will sound, while the indicator stays on.This signaling process is summarized as follows:

                  TABLE 1                                                         ______________________________________                                        Resistance      Indicator Audible                                             Range           170 Status                                                                              Alarm 130                                           ______________________________________                                        0-150 Kilohms   On        On                                                  200 Kilohms-    On        Off                                                 2.5 Megohms                                                                   3.0 Megohms     Off       Off                                                 to Open                                                                       ______________________________________                                    

Under the second method, the wrist strap 120, attached to the person 11is plugged into test terminal 160 and the person can now touch theconductive benchtop mat 20 with the palm of his or her hand. Indicator170 will be activated to indicate that both the wrist strap 120 and thebenchtop mat 20 are conducting and properly grounded. The conductivefloor mat 25 may also be tested in the same manner by placing the palmof the hand on the floor mat 25. Under this second method, if the"Strap/Conductivity" test indicator 170 fails to become activated thenthe wrist strap 120 may be defective and should be tested according tothe procedure set forth above. If the wrist strap 120 passes, then theground wires 17 and 27 for the conductive bench mat 20 or the floor mat25 may be broken, disconnected or deteriorated and they should bechecked and repaired if necessary. If the ground wires 17 and 27 arefully operative, then the resistance of mats 20 and 25 may havedeteriorated and should be checked with the conductivity test as setforth in the next section. This test may also be conducted with thewrist strap 120 plugged into terminal 186 for equipment where mats 20and 25 have very high resistance--in excess of the sensitivity of thewrist strap input. With the wrist strap 120, plugged in to terminal 186,normally used with surface resistance probe 180 described below, a totalresistance of up to 1000 Megohms may be measured, whereas terminal 160is limited to 2.5 Megohms.

It can be observed that the electrostatic system monitor 10 of thepresent invention enables the user 11 to periodically ascertain theintegrity of his or her wrist strap 120, the integrity of the benchconductive mat 20 and floor conductive mat 25, or other static controlequipment. In operation this should be done several times a day such asjust after starting work, each break, and after lunch.

3. Conductivity Test

The electrostatic system monitor 10 of the present invention is alsoable to quickly measure the conductivity of the workbench mat 20 and thefloor mat 25. It can also be used to ascertain the conductivity of otheritems such as conductive tote boxes, bags, seat covers and lab coats.

A surface resistance probe 180 is provided which is interconnected overcables 182 and 184 to conductivity test terminals 186 and 188respectively. Suitable mating connectors are provided on opposing endsof cables 182 and 184. The surface resistance probe 180, as will bediscussed in greater detail subsequently, contains two one inch steelcubes designed to measure surface resistance. In operation, when thesurface resistance probe 180 is interconnected to the electrostaticsystem monitor 10 of the present invention over cables 182 and 184, theprobe 180 measures surface resistance. "Adequate Conductivity" is a termdefined herein to mean less than 1000 Megohms per square inch surfaceresistance. That amount of conductivity is adequate for the mats andother conductive surfaces and the user 11 is assured that properelectrostatic discharge conditions are being met. With adequateconductivity, the "Strap/Conductivity" Test indicator 170 will becomeactivated and the audible alarm 130 will sound to both visually andaudibly indicate that the surface has, in fact, adequate conductivity.If the conductivity of the surface is between approximately 1000 and3000 Megohms per square inch surface resistance, only the visualindicator 170 will become activated. Finally, for surface conductivitiesof greater than approximately 3000 Megohms per square inch surfaceresistance, neither the visual indicator 170 nor the audible alarm 130will be activated. This is summarized below:

                  Tab1e 2                                                         ______________________________________                                        Surface        Visual     Audible                                             Resistance     Indicator 170                                                                            Indicator 130                                       ______________________________________                                        0-1000 Megohms On         On                                                  1000-3000      On         Off                                                 Megohms                                                                       Greater Than   Off        Off                                                 3000 Megohms                                                                  ______________________________________                                    

In conclusion, the present invention provides an additional probe 180for quickly ascertaining whether or not the conductive mats 20 and 25 orother conductive items in the work station environment have adequateconductivity to be effective.

In another embodiment of the present invention, the indicator 170 onlywill remain on for resistances ranging from approximately 100 Kilohms(or any other set limit) to 1000-3000 Megohms, while materials with lessthan 100 Kilohms (or other set limit), considered too conductive forcontrolled discharge of electrostatic charges, will activate bothindicator LED 170 and alarm 130. The advantage of this embodiment, isthat an alarm signal always indicates a failure or dangerous condition.

4. Soldering Iron Test

FIG. 1 further illustrates a grounded soldering iron 190 which isconnected over electrical cord 192 to a conventional outlet 40. The user11 uses the grounded soldering iron 190 on a typical work piece 200. Ifthe tip 202 of the soldering iron 190 become ungrounded, either due tocorrosion or due to a broken ground wire, a high AC voltage may beinduced in the tip 202 from the field surrounding the heating element.It is to be noted that SCR controlled soldering irons are especiallysusceptible to this type of problem. Indeed, voltage spikes of severalhundred volts are not uncommon and such voltages destroy electrostaticsensitive work pieces 200 such as those that include PMOS and CMOScircuitry. Under the teachings of the present invention, theelectrostatic system monitor 10 of the present invention will test thesoldering iron 190 for the presence or absence of AC voltage ortransient spikes on the tip 202. The user 11 periodically manuallytouches the tip 202 of the iron 190 to terminal 210. If AC voltage ortransient spikes exceed approximately five volts RMS or seven voltspeak, or any other preset voltage, into an approximately 30 Megohms, orany other, specified load, then the tip 202 and the iron 190 isdefective and should not be used. If this occurs, an indicator 220 isvisually activated and the audible alarm 130 will sound. A safesoldering iron 190 with a properly grounded tip 202 will not activateeither the visual indicator 220 or the audible alarm 130.

Therefore, in use and operation, the user 11 periodically touches thetip 202 of his or her soldering iron 190 to terminal 210. This touchingshould occur several times a day as, for example, when the worker startsup at the work station and after each break period.

While the above represents tests on conventional static controlequipment such as mats, wrist straps, and soldering irons, the teachingsof the present invention can be applied to one or a plurality of theabove tests as well as tests for heel straps, conductive containers andthe like to monitor their performance.

5. Central Monitor

As shown in FIG. 2, a number of electrostatic system monitors 10 of thepresent invention can be interconnected over cables 230 to a centralmonitor 240. Each electrostatic system monitor 10 contains a remotecontrol output which is connected to cable 230 into the central monitor.In this fashion, the central monitor can monitor the operation of eachelectrostatic system monitor 10. The electrostatic system monitor 10normally provides a low current output of zero volts to its line fornormal operation and twelve volts DC over that same line for an alarmindication. It is contemplated that the central monitor 240 can containcircuitry to monitor up to one hundred static system monitors, 10, andtherefore one hundred work stations. Additional static system monitors,or work stations, can be monitored by using additional central monitors.

DETAILED CIRCUIT DESCRIPTION

In FIG. 3, the electronic schematic for the circuitry contained withinstatic system monitor 10 is set forth. Power is delivered into thecircuitry over cable 50 when plug 300 is plugged into electrical outlet40. Electrical ground appears on line 302 and alternating current poweris delivered over lines 304 and 306. The power on lines 304 and 306accesses transformer 308 and power over line 306 is delivered through aprotective fuse 310. The secondary of transformer 308 has a center tapwhich is connected to ground line 312 and to connector 90 which is theearth ground connector. Hence, and with reference back to FIG. 1, earthground is delivered by means of cable 60 from a water pipe 80 intoconnector 90 and to line 312 which provides earth ground and the groundconnection for the circuitry shown in FIG. 3. Line 312 is also deliveredinto the wrist strap connector 150 for delivery through interconnectingcable 131 to the wrist strap 120 of user 11. Hence, earth ground is alsoprovided to the wrist strap 120 over line 312 and is further provided toconnector 188 which is the ground connection over cable 184 to thesurfaces resistance probe 180. Earth ground appearing on line 312 isalso delivered into connector 314 which is the connector shown in FIG. 2for connection to the central monitor 240. Hence, the central monitor240 also receives earth ground. As can be appreciated, the use of earthground is important in minimizing the effects of static electricity.

At this point, it should be pointed out that if a customer preferselectrical ground to earth ground the present invention can optionallyprovide electrical ground to line 312 in the following fashion. Anoptional connection 316 is inserted to provide a conducting path betweenline 302 and line 312. In this arrangement, cable 60 is not plugged intoconnector 90 and with connection 316 in place, electrical ground isprovided over line 312. It is to be expressly understood that this is anoptional approach and that either connection 316 for electrical groundor cable 60, connected to a water pipe 80, for earth ground may be used.

Returning to the output of the secondary of transformer 308, thesecondary accesses two diodes to provide full wave rectification and toprovide approximately fifteen volt DC output. In the preferredembodiment, diodes 316 are conventionally available as models 1N4003.The full wave rectified output is delivered over lines 318 and isdelivered to filter capacitor 320, connected to earth ground 312, and isalso delivered through resistor 322 to a visual indicator 121. In thepreferred embodiment, capacitor 320 is a 470 microfarad capacitor andresistor 322 is 150 ohms. The visual indicator 121 is a conventionallight emitting diode (LED). Across LED 121 and resistor 322 is connectedin parallel a resistor 324 having one end connected to line 318 and itssecond end connected to line 320. Resistor 324 preferably is 120 ohms.Line 320 is connected to Zener diode 326, preferably 1N4742A, which hasits other end connected at earth ground 312.

Essentially the circuit involving the transformer 308, the diodes 316,Zener diode 326, resistors 322 and 324, and LED 121 function to providea regulated twelve volt DC level on line 320. It is to be expresslyunderstood that other power supply circuits can be designed to providethe same type of regulated voltage on line 320 as herein disclosed. Aslong as power is provided to the circuit the light 121 is activated.

1. Ground Continuity Circuit

The ground continuity function, as previously discussed, permits acontinuous measurement of the quality of both the earth ground appearingon line 312 and the electrical ground appearing on line 302. This isaccomplished as follows. The twelve volts DC appearing on line 320 isdelivered through resistor 328 to electrical ground on line 302.Resistor 328 is preferably 3.6 kilohms. resistor. The base of transistorQ8, connected to line 330, is delivered through resistor 332 which hasits other end connected at electrical ground 302. The emitter of Q8 isconnected to earth ground appearing on line 312. The collector oftransistor Q8 is connected to line 334 which is in turn connected toalarm 130 and the other side of alarm 130 is connected to the twelvevolts DC on line 320. In addition, the GROUND OK light 110 has one endconnected to earth ground on line 312 and its opposing end connectedthrough resistor 336 to power on line 320 and is further connectedthrough diode 338 to line 334. In the preferred embodiment, transistorQ8 is a Model No. 2N3904 which is conventionally available, visualindicator 110 is a light emitting diode, resistor 336 is 620 ohms, anddiode 338 is conventionally available as Model No. 1N4146.

Transistor Q8 essentially measures the resistance difference between theearth ground on line 312 and the electrical ground on line 302. Forexample, as the resistance between earth ground on 312 and electricalground on 302 increases, a voltage divider occurs between resistor 328and resistor 332 and this ground resistance thereby causing more currentto be delivered through resistor 332 and to turn on transistor Q8. Astransistor Q8 turns on, the potential on line 334 moves towards line312, which is also earth ground.

Now, as long as there is no ground resistance being developed,transistor Q8 is off and indicator 110 is continually lit by deliveringcurrent through resistor 336.

Hence, as long as the grounds are properly functioning, the indicatorlight 110 is always on. However, if a resistance develops between earthground and electrical ground, an electrical signal is issued by thecircuit when transistor Q8 turns on and, in the preferred embodiment, itwill turn on when the resistance increases above the predeterminedvalue, 100 ohms, or should either earth ground or electrical groundbecome open circuited. When Q8 turns on under these conditions, thevoltage on line 334 moves towards ground on line 312 causing diode 338to turn on and short circuiting or turning off indicator light 110. Thisalso increases the voltage across the alarm 130 which then becomesactivated. Hence, when the total resistance between the two groundsbecomes greater than 100 ohms, or either ground becomes open circuited,indicator light 110 turns off and alarm 130 turns on.

It is to be expressly understood that in the preferred embodiment theturn on point is approximately 100 ohms but this can be varied byaltering the values for resistors 328 and 332.

Transistor Q6 provides snap action hystersis for transistor Q8 so thatboth the alarm 130 and the indicator 110 turn on and off abruptly anddistinctly rather than gradually. Transistor Q6 has its emitterconnected to the DC power on line 320 and has its base connected throughresistor 352 to DC power 320 and its base is further connected throughresistor 354 to line 334. The collector of transistor Q6 is connected toline 344 and is further connected through resistor 356 to earth groundon 312. In the preferred embodiment, transistor Q6 is a conventionalModel 2N3906 transistor, resistor 352 is 3.3 Kilohms, resistor 354 is 15Kilohms, and resistor 356 is 10 Kilohms. In operation, as transistor Q8turns on and as the potential on line 334 moves towards ground, thevoltage across resistor 354 increases to deliver more current to thebase of Q6 thereby rapidly turning on Q6 which provides more voltage toline 344. This sends a current through resistors 346 and 406, (56 Kilohmand 1.5 Kilohm) into the base of Q8, thereby accellerating the turn-onof Q8. With Q8 and Q6 on, the twelve volts on Line 320, sends a currentthrough resistor 342 to the output jack 314. Hence, in an alarmcondition, the twelve volt signal is delivered to the central monitor240 and in normal conditions, ground is essentially applied.

In summary, the ground integrity is monitored continuously and in theevent of any resistance changes or open circuits in the earth orelectrical grounds, the system rapidly functions to sound an alarm 130,to send that signal to a central monitor 240 and to extinguish indicatorlight 110.

2. Soldering Iron Test Circuit

As previously discussed, the soldering iron function permits the testingof the soldering iron 190 to ascertain that it is free of any damagingvoltage on the tip 202 due to bad grounding, broken wires, corrosion orother defects. The circuitry to be described prevents such damage bymeasuring the actual voltage on the tip 203 when the tip 202 is touchedto connector 210. If voltage on the tip 202 is greater than apredetermined voltage, such as five volts RMS or seven volts peak into aload of approximately thirty Megohms, the indicator 220 will becomeactivated and the alarm 130 will sound. A safe iron tip 202 maintainsthe audible alarm 130 and the visual indicator 220 in an off state.

In operation, any induced AC signal on the iron tip 202 is deliveredinto terminal 210 and further delivered through resistor 358 into line360. Line 360 is connected through resistor 362 and 364 to earth ground312 and line 360 is further connected through capacitor 366 and diode368 to capacitor 374 and line 370; the other end of capacitor 374 isconnected to ground 312. In the preferred embodiment, resistors 358,362, and 364 are each ten Megohms and the three resistances combine toan input load of 30 Megohms. Capacitor 366 and 374 are 0.01 microfaradsand diode 368 is preferably a Model No. 1N4148.

Between capacitor 336 and diode 368 is another diode 372 which isconnected to ground 312. The diode, 372, is preferably Model No. 1N4148.Capacitors 366 and 374 and diodes 368 and 372 act as a voltage doublingrectifier to rectify the AC voltage appearing on line 360 and placingthe recitified D.C. voltage on line 370. Resistors 358, 362, and 364,totaling 30 Megohms, must be a high impedance because the inducedvoltage, appearing on the tip 202 of the iron 190 would be severelyreduced by a low impedance load. The rectified and doubled input voltageappearing on line 370 is then delivered to the base of transistor Q4which has its emitter coupled to the base of transistor Q5. Thecollectors of transistor Q4 and Q5 are tied together and are deliveredthrough resistor 376 to power on line 320. The emitter of transistor Q5is delivered through resistor 378 to the base of transistor Q7. In thepreferred embodiment, transistors Q4 and Q5 are conventionally availableas Model Nos. 2N3904 and are interconnected as a current amplifier.Resistor 380 is connected in parallel across the base input totransistor Q4 and the emitter output of transistor Q5. The output ofresistor 378 is delivered through resistor 382 to earth ground on 312and, as mentioned, is delivered into the base of transistor Q7. In thepreferred embodiment, resistor 376 is one Kilohm, resistor 378 is 2.2Kilohms, resistor 380 is ten Megohms, resistor 382 is one Megohm andtransistor Q7 is conventionally available as Model No. 2N3904. Thecollector of transistor Q7 is tied back to previously discussed line 334and the emitter of transistor Q7 is delivered through a light emittingdiode 220 to earth ground 312.

This circuit functions as follows. An AC signal appearing on connector210 is delivered into a high impedance load and then the portion of thisAC voltage appearing on line 360 is rectified and doubled when itreaches line 370. At this point, the AC signal has been rectified into adirect current voltage delivered into the base of Q4. The rectifiedsignal is further amplified, and the gain is determined in part by thevalues of resistors 378, 380 and 382 to turn on transistor Q7 if the ACvoltage on connector 210 exceeds five volts RMS or seven volts AC peak.To summarize, when Q4 turns on, Q5 turns on and an amplified signal isdelivered into the base of transistor Q7. In the preferred embodiment,approximately 2.7 volts at the base is required to turn on transistorQ7. With transistor Q7 turned on, light emitting diode 220 becomesactivated and turns on. Furthermore, the turning on of transistor Q7, aswith the turning on of transistor Q8, moves the potential on line 334toward ground, to a potential of approximately two volts and therebyactivating the alarm 130. Since the voltage on Line 334 does notdecrease below two volts, LED 110 will not be turned off, even thoughalarm 130 is activated. In a similar fashion, and as previouslydiscussed, transistors Q6 and Q8 provides hystersis to quickly turn onand off the alarm and to provide a twelve volt signal on jack 314 fordelivery to the central monitor 240.

It can be readily observed that the soldering iron test shares circuitrywith the continuous ground monitor circuit. Again, the aforesaidpreferred values for the components can be modified to provide forgreater or lesser sensitivity to the induced voltage appearing on thetip 202 of the iron 190, and delivered to terminal 210.

3. Wrist Strap Circuit

As previously discussed, the wrist strap 120 is periodically manuallytouched to connector 160 by the user. Table 1 sets forth the threeconditions for the wrist strap test. Terminal 160 is connected throughresistor 384 to line 397. Line 397 connects to power on line 320 throughresistor 386 and to line 396 through diode 394. Line 396 connects to thebase of transistor Q2, to one end of capacitor 398, to the emitter oftransistor Q1 and to one end of resistor 410. Terminal 160 is alsoconnected through resistor 388, through trim-potentiometer 389 andthrough diode 390 to the collectors of transistors Q1, Q2 and Q3 andthrough resistor 392 and light-emitting diode indicator 170 to earthground 312. In the preferred embodiment, resistor 384 is 1.5 Megohms,resistor 386 is 680 Kilohms, resistor 388 is 62 Kilohms,trim-potentiometer 389 is 50 Kilohms, diodes 390 and 394 are Model Nos.1N4148, capacitor 398 is 0.1 microfarad and resistor 392 is 510 ohms.Because of this voltage-dividing arrangement, the terminal 160 normallysits at twelve volts DC. Hence, it can be observed through reference toTable 1, that depending upon the resistance from connector 160 throughthe wrist strap 120 through cable 131 to ground at terminal 150 thevoltage level at terminal 160 can significantly vary. For example, ifthe wrist strap 120 functions properly, then the resistance which willbe encountered between terminal 150 and terminal 160 is between 200Kilohms and 2.5 Megohms. When this occurs, a voltage drop range of lessthan twelve volts must occur across resistors 384 and 386. Line 397 mustdrop below approximately ten volts to turn on diode 394, which will turnon transistors Q2 and Q3 and activate LED 170. In the preferredembodiment, transistors Q2 and Q3 are conventionally available as ModelNos. 2N5086. The diode 394 acts as a level triggered switch to turn onwhen the voltage at line 397 drops below approximately ten volts.

The emitter of transistor Q2 is connected through resistor 400 andtrim-potentiometer 401 to power on line 320 and the emitter is furtherconnected to the base of transistor Q3. The outputs of transistors Q1,Q2 and Q3 and tied together on line 402 which are delivered intoresistor 404. Resistor 404 is in turn connected to line 348 and isdelivered through resistor 406 to line 330 at the base of transistor Q8.As previously mentioned, line 402 is further connected back throughdiode 390, trim-potentiometer 389 and resistor 388 to terminal 160. Inthe preferred embodiment, resistor 400 is 2.0 Kilohms,trim-potentiometer 401 is 25 Kilohms, resistor 404 is 4.7 Kilohms andresistor 406 is 1.5 Kilohms. Trim-potentiometers 401 and 389 enablesetting the specified test levels more precisely than fixed resistorsand may be substituted with resistors which are selected in productiontest.

The wrist strap circuit functions as follows and with specific referenceback to Table 1. When the wrist strap 120 is touched to the terminal 160and the safe resistance range of 200 Kilohms to 2.5 Megohms is measured,only the indicator light 170 is activated, since line 402 will rise onlyuntil diode 390 turns on, at which point it will attempt to turntransistor Q2 off. Equilibrium is reached at this point, and theresistance between terminals 160 and 150 must decrease below 150 Kilohmsfor line 402 to rise in voltage above this point to activate alarm 130through resistor 404.

Hence, as long as the voltage being delivered by the voltage dividerformed by resistors 384 and 386 to line 397 and diode 394 is in therange of approximately ten volts thereby signaling a safe resistance,diode 394 turns on, thereby turning on transistor Q2 which in turn turnson transistor Q3 delivering current through resistor 392 from line 402to activate the light-emitting diode 170. It is to be noted by referenceto Table 1, that as long as the resistance back through the wrist strapis between approximately 2.5 Megohms and 200 Kilohms, the light-emittingdiode 170 is turned on, and the alarm 130 is off. If the resistance isgreater than approximately three Megohms signaling an increase inresistance above the predetermined resistance range, the voltage at line397 will rise above ten volts and turn off diode 394 and transistors Q2and Q3. Therefore, indicator 170 and alarm 130 are held in the offstate. When the resistance between terminals 160 and 150 drops below 150Kilohms signaling a decrease in resistance below the predeterminedresistance range, transistors Q2 and Q3 are conducting more current,permitting the collector of transistor Q3 to rise beyond the equilibriumpoint, determined by the turn-on of diode 390, with some of this extracurrent going through diode 390, trim-potentiometer 389 and resistor388, forming a feedback loop through resistor 384 and diode 394, back tothe base of transistor Q2. This reduces the gain of amplifiers Q2 and Q3and, therefore, requires a resistance of less than 150 Kilohms betweenterminals 160 and 150 for transistors Q2 and Q3 to saturate at a voltageon line 402, sufficiently large to turn on alarm 130, through resistor404 and resistor 406, feeding current into the base of transistor Q8and, as before, indicator 170 also remains activated. Again, when alarm130 is activated, a signal is delivered to connector 314 for deliveryover cable 230 to the central monitor 240.

Hence, transistors Q2 and Q3 function as an amplifier stage and a driverfor the indicator light 170 and as a driver for alarm 130 through Q8,after diode 390 turns on and causes a reduction in gain for theamplifier consisting of Q2 and Q3. The limits for setting the voltagedrop range on terminal 160, is controlled by resistors 384, 386, and 388and trim-potentiometer 389. Again, it is to be noted that substantialsharing of the circuitry occurs with other tests, and that diode 390functions as a level-triggered switch, which, when turned on, lowers thegain of the amplifier consisting of transistors Q2 and Q3, todiscriminate narrowly between resistance values between terminals 160and 150 of more or less than 200 Kilohms, by causing an abrupt change inthe voltage level on line 402 which causes alarm 130 to turn on abruptlyfor resistances of less than 150 Kilohms and off for resistances greaterthan 200 Kilohms.

4. Conductivity Test Circuit

As previously discussed, the conductivity test functions to make surfaceresistance measurements on static control materials such as theconductive mat 20 on workbench 15 and the conductive floor mat 25.

The conductivity test requirements are essentially set forth in Table 2,above, and operates on the same principles and shares substantially thesame circuitry as for the wrist strap test. In normal operation,terminal 188 is maintained at earth ground from line 312. Terminal 186,however, is essentially maintained at twelve volts through resistor 408and resistor 410 which is connected over line 396 to the emitter oftransistor Q1 and the base of transistor Q2. Resistor 408 limits shortcircuit current and resistor 410 shunts any collector-to-base leakage intransistor Q1. Terminal 186 delivers a signal through resistor 408 toline 412 into the base of transistor Q1, which is decoupled againstnoise through capacitor 414 to power on line 320. Capacitor 398 alsodecouples the base of transistor Q2, to minimize noise pick-up. In thesituation where the resistance is greater than approximatly 3000 Megohmsper square inch signaling less than adequate conductivity, the voltageon connector 186 is maintained at a DC level greater than ten volts andtransistor Q1 does not turn on. Hence, the indicator 170 and the alarm130 will not become activated. When this occurs, conductive materialssuch as conductive mat 20 or 25 are defective. On the other hand,materials having conductivity which is less than 1000 Megohms per squareinch is indicated by both the visual indicator 170 and the audible alarm130 becoming activated. In that event, ground through an adequately lowresistance appears on terminal 186 signaling more than adequateconductivity and causing transistors Q1, Q2 and Q3 to activate, all ofwhich comprises a three stage amplifier. This in turn provides power tolight emitting diode 170 and to alarm 130 as previously discussed. Forthe situation of adequate conductivity having a predeterminedconductivity range of resistance between 1000 Megohms per square inchand 3000 Megohms per square inch, transistor Q3 is not driven to asufficiently saturated state to activate the alarm through resistor 404.Hence, only the light emitting diode 170 is activated.

5. Surface Resistance Probe

In FIG. 4 is set forth the preferred embodiment for the surfaceresistance probe of the present invention. The probe 180 includes ahandle or bridge 450, two rubber bumpers 460, and two contact blocks470. The handle is made from a highly insulating plastic of extremelyhigh resistance such as more than 10¹⁴ ohms.

The contact blocks 470 are one inch square cubes that are machined fromsteel and coated with nickel plate. The bottom surface 472 of each cubeis machined or ground flat for uniform surface contact. The cubes 470each have a drilled hole 474 for receiving an electrical connector fromcables 182 and 184. On the upper surface of the cube is a drilled hole476 which is threaded and which is receptive of a threaded stud 462downwardly extending from each rubber bumper 460. Each insulating bumper460 is made from flexible rubber and the metal stud 462 is molded intothe rubber bumper 460. Each rubber bumper 460 also has a threaded metalinsert 464 which is molded into the rubber bumper and is receptive ofbolts 452 which are passed through drilled, milled, or molded holes 454contained within the handle 450. Note that insert 464 and threaded stud462, contained within the rubber bumpers 460, are not in electrical ormechanical contact and permit stud 462 to flex relative to insert 464.When assembled, the bolts 452 firmly engage the threaded inserts 464 tohold the rubber bumpers 460 against the handle 450 and each rubberbumper in turn by means of its threaded stud 462 engages drilled andthreaded hole 476 in each cube 470 to firmly hold each cube in position.

This is best shown by the example set forth in FIG. 5. For example, if aconductive mat 20 contains a rise 500, because of the rubber bumpers orshock absorbers 460, the surface resistance probe 180 of the presentinvention is capable of firmly engaging the uneven surface 20 to makethe conductivity check. The spacing between the inside surfaces of eachcube 470 is exactly one inch apart, such that surface resistance "persquare" is being measured. It is assumed and required that some downwardpressure is exerted on handle 450, to assure good surface contact withthe material being measured. Surface resistance "per square" is oftenspecified to be measured with five pounds of pressure on the probe.

It should be expressly understood that the test values and limits setforth and described throughout this specification, are strictly choices,based on typical usage of the present invention, and that these limitsand values may be changed over a wide range by changing one or more ofthe component values that set those values and limits. It should also beexpressly noted that any diode or transistor type, or any other partscalled out, may be substituted with equivalent parts without affectingthe present invention. This includes the use of integrated circuits inplace of discrete transistors, or part integrated circuits and partdiscrete transistors, without altering the functions, performance andintent of the present invention.

Although the present invention has been described with a certain degreeof particularity, it is to be expressly understood that the presentdisclosure has been made by way of example and that changes in detailsof components and structure, as set forth in the preceding paragraph,may be made without departing from the spirit of the invention and asfound in the following claims.

I claim:
 1. An electrostatic system monitor for monitoring the operationof static control equipment located at a user's workstation, said staticcontrol equipment including (a) conductive surfaces such as a conductiveworkbench mat and a conductive floor mat, (b) a grounded tip solderingiron, and (c) a conductive wrist strap on the wrist of a user, saidconductive surfaces and said wrist strap being connected to groundconnection, said monitor and soldering iron being connected toelectrical AC power having an electrical ground, said electrostaticsystem monitor comprising:means connected to said ground and to saidelectrical ground for continuously sensing the low impedance continuitybetween said ground and said electrical ground, said sensing means beingcapable of issuing a loss-of-continuity signal in the event said lowimpedance exceeds a predetermined resistance value, means responsive tothe manual contact with said conductive wrist strap for determining theresistance of said conductive wrist strap, said determining means beingcapable of producing (a) a safe resistance signal when the determinedresistance is in a predetermined resistance range, (b) aloss-of-resistance signal when the determined resistance is below saidpredetermined resistance range, and (c) a gain-of-resistance signal whenthe determined resistance is above said predetermined resistance range,a surface resistance probe for detecting the surface resistance of saidconductive surfaces when said probe is in contact with said surfaces,means connected to said surface resistance probe for selectivelymeasuring the conductivity of said conductive surfaces, said measuringmeans being capable of generating (a) an adequate conductivity signalwhen the surface resistance is in a predetermined resistance range, (b)an unsafe resistance signal when the surface resistance is below saidpredetermined resistance range, and (c) an inadequate conductivitysignal when the surface resistance is above said predeterminedresistance range, and means responsive to a periodic manual contact withthe tip of said soldering iron for detecting induced AC voltage, saiddetecting means being capable of producing an unsafe voltage signal whensaid induced AC voltage exceeds a preset voltage.
 2. The electrostaticsystem monitor of claim 1 further comprising:a first visual indicatorreceptive of said loss-of-continuity signal from said sensing means forindicating that the impedance between said ground and said electricalground has exceeded said predetermined resistance value, and an audiblealarm receptive of said loss-of-continuity signal from said sensingmeans for audibly indicating that the impedance between said ground andsaid electrical ground has exceeded said predetermined resistance value.3. The electrostatic system monitor of claim 2 wherein saidpredetermined resistance value is substantially 100 ohms.
 4. Theelectrostatic system monitor of claim 2 further comprising:a secondvisual indicator receptive of said safe resistance andloss-of-resistance signals from said determining means for indicatingthat the resistance is less than or within said predetermined resistancerange, and said audible alarm being further receptive of saidloss-of-resistance signal from said determining means for indicatingthat the resistance is less than that of said predetermined resistancerange.
 5. The electrostatic monitor of claim 4 wherein saidpredetermined resistance range is between substantially 200 Kilohms and2.5 Megohms.
 6. The electrostatic system monitor of claim 4 furthercomprising:said second visual indicator being further receptive of saidadequate and unsafe resistance signals from said measuring means forindicating that the surface resistance is less than or within saidpredetermined conductivity range, and said audible alarm being furtherreceptive of said unsafe resistance signal from said measuring means forindicating that the surface resistance is less than that of saidpredetermined resistance range.
 7. The electrostatic system monitor ofclaim 6 wherein said predetermined adequate conductivity range isbetween 10 Megohms per square inch and 3000 Megohms per square inch. 8.The electrostatic system monitor of claim 2 further comprising:a thirdvisual indicator receptive of said unsafe voltage signal from saiddetecting means for indicating that said soldering iron is not safe touse, and said audible alarm being further receptive of said unsafevoltage signal from said detecting means for indicating that saidsoldering iron is not safe to use.
 9. The electrostatic system monitorof claim 8 wherein said preset voltage is substantially five volts RMS.10. The electrostatic system monitor of claim 1 wherein said detectingmeans comprises:a high impedance load receptive of said induced ACvoltage from said soldering iron tip, means receptive of said AC voltageacross said high impedance load for rectifying said voltage, saidrectifying means being further capable of doubling said AC voltage, andmeans receptive of said rectified and doubled voltage from saidrectifying mean for issuing said unsafe voltage signal when saidrectified and doubled voltage exceeds said preset voltage.
 11. Theelectrostatic system monitor of claim 1 wherein said surface resistanceprobe comprises:a handle made from highly insulating material, a pair offlexible and insulating bumpers connected to the underside of saidhandle on opposing ends thereof, and a pair of conductive contact blocksconnected on the underside of said bumpers, said contact blocks beingcapable of adjusting to uneven contact surfaces on said mats by means ofthe flexing action of said bumpers, said bumpers further holding saidcontact blocks a predetermined distance apart.
 12. The electrostaticsystem monitor of claim 1 further comprising:means operative upon theactivation of said audible alarm for generating an alarm signal, andmeans located remote from said generating means and receptive of saidalarm signal for monitoring when said alarm signal is generated.
 13. Anelectrostatic system monitor for monitoring the operation of staticcontrol equipment at a user's workstation, said static control equipmentincluding (a) conductive surfaces such as a conductive workbench mat anda conductive floor mat, (b) a grounded tip soldering iron, and (c) aconductive wrist strap on the wrist of a user, said conductive surfacesand wrist strap being connected to a ground connection, said monitor andsoldering iron being connected to electrical AC power having anelectrical ground, said electrostatic system monitor comprising:meansconnected to said ground and to said electrical ground for continuouslysensing the low impedance continuity between said ground and saidelectrical ground, said sensing means being capable of issuing aloss-of-continuity signal in the event said low impedance exceeds apredetermined resistance value, a first indicator receptive of saidloss-of-continuity signal from said sensing means for indicating thatthe impedance between said ground and said electrical ground hasexceeded said predetermined resistance value, means responsive to amanual contact with said conductive wrist strap for determining theresistance of said conductive wrist strap, said determining means beingcapable of producing (a) a safe resistance signal when the determinedresistance is in a predetermined resistance range, (b) aloss-of-resistance signal when the determined resistance is below saidpredetermined resistance range, and (c) a gain-of-resistance signal whenthe determined resistance is above said predetermined resistance range,a surface resistance probe for detecting the surface resistance of saidconductive surfaces when said probe is in contact with said conductivesurfaces, means connected to said surface resistance probe forselectively measuring the conductivity of said conductive surfaces, saidmeasuring means being capable of generating (a) an adequate conductivitysignal when the surface resistance is in a predetermined resistancerange, (b) an unsafe resistance signal when the surface resistance isbelow said predetermined resistance range, and (c) an inadequateconductivity signal when the surface resistance is above saidpredetermined resistance range, a second indicator receptive of saidsafe resistance and loss-of-resistance signals from said determiningmeans and of said adequate and unsafe resistance signals from saidmeasuring means for becoming activated upon the receipt of any one ofthe aforesaid signals, means responsive to a manual contact with the tipof said soldering iron for detecting induced AC voltage, said detectingmeans being capable of producing an unsafe voltage signal when saidinduced AC voltage exceeds a preset voltage, a third indicator receptiveof said unsafe voltage signal from said detecting means for indicatingthat said soldering iron is not safe to use, and an alarm receptive of(a) said loss-of-continuity signal from said sensing means, (b) saidloss-of-resistance signal from said determining means, (c) said unsaferesistance signal from said measuring means, and (d) said unsafe voltagesignal from said detecting means for becoming activated upon the receiptof any one of the aforesaid signals.
 14. The electrostatic systemmonitor of claim 13 further comprising:means operative upon theactivation of said audible alarm for generating an alarm signal, andmeans located remote from said generating means and receptive of saidalarm signal for monitoring when said alarm signal is generated.
 15. Anelectrostatic system monitor for monitoring the operation of staticcontrol equipment at a user's workstation, said static control equipmentincluding (a) conductive surfaces such as a conductive workbench mat anda conductive floor mat, (b) a grounded tip soldering iron, and (c) aconductive wrist strap on the wrist of a user, said conductive surfacesand wrist strap being connected to a ground connection, said monitor andsoldering iron being connected to electrical AC power having anelectrical ground, said electrostatic system monitor comprising:meansconnected to said ground and to said electrical ground for continuouslysensing the low impedance continuity between said ground and saidelectrical ground, said sensing means being capable of issuing aloss-of-continuity signal in the event said low impedance exceeds apredetermined resistance value, a first indicator receptive of saidloss-of-continuity signal from said sensing means for indicating thatthe impedance between said ground and said electrical ground hasexceeded said predetermined resistance value, means responsive to amanual contact with said conductive wrist strap for determining theresistance of said conductive wrist strap, said determining means beingcapable of producing (a) a safe resistance signal when the determinedresistance is in a predetermined resistance range, (b) aloss-of-resistance signal when the determined resistance is below saidpredetermined resistance range, and (c) a gain-of-resistance signal whenthe determined resistance is above said predetermined resistance range,a second indicator receptive of said safe resistance andloss-of-resistance signals from said determining means for becomingactivated upon the receipt of any one of the aforesaid signals, meansresponsive to a manual contact with the tip of said soldering iron fordetecting induced AC voltage, said detecting means being capable ofproducing an unsafe voltage signal when said induced AC voltage exceedsa preset voltage, a third indicator receptive of said unsafe voltagesignal from said detecting means for indicating that said soldering ironis not safe to use, and an alarm receptive of (a) saidloss-of-continuity signal from said sensing means, (b) saidloss-of-resistance signal from said determining means, and (c) saidunsafe voltage signal from said detecting means for becoming activatedupon the receipt of any one of the aforesaid signals.
 16. Theelectrostatic system monitor of claim 15 further comprising:meansoperative upon the activation of said audible alarm for generating analarm signal, and means located remote from said generating means andreceptive of said alarm signal for monitoring when said alarm signal isgenerated.
 17. An electrostatic system monitor for monitoring theoperation of static control equipment such as a wrist strap at a user'sworkstation, said static control equipment being connected to a groundconnection, said monitor being connected to electrical AC power havingan electrical ground, said electrostatic system monitor comprising:meansconnected to said ground and to said electrical ground for continuouslysensing the low impedance continuity between said ground and saidelectrical ground, said sensing means being capable of issuing aloss-of-continuity signal in the event said low impedance exceeds apredetermined resistance value, a first indicator receptive of saidloss-of-continuity signal from said sensing means for indicating thatthe impedance between said ground and said electrical ground hasexceeded said predetermined resistance value, means responsive to amanual contact with said static control equipment for determining theresistance of said equipment, said determining means being capable ofproducing (a) a safe resistance signal when the determined resistance isin a predetermined resistance range, (b) a loss-of-resistance signalwhen the determined resistance is below said predetermined resistancerange, and (c) a gain-of-resistance signal when the determinedresistance is above said predetermined resistance range, a secondindicator receptive of said safe resistance and loss-of-resistancesignals from said determining means for becoming activated upon thereceipt of any one of the aforesaid signals, and an alarm receptive ofsaid loss-of-continuity signal from said sensing means and saidloss-of-resistance signal from said determining means, for becomingactivated upon the receipt of any one of the aforesaid signals.
 18. Amethod for monitoring the operation of static control equipment at auser's workstation, said static control equipment including (a)conductive surfaces such as a conductive workbench mat and a conductivefloor mat, (b) a grounded tip soldering iron, and (c) a conductive wriststrap on the wrist of a user, said conductive surfaces and wrist strapbeing connected to a ground connection, said soldering iron beingconnected to electrical AC power having an electrical ground, saidmethod comprising the steps of:continuously sensing the low impedancecontinuity between said ground and said electrical ground, issuing aloss-of-continuity signal in the event said low impedance exceeds apredetermined resistance value, activating a first indicator in responseto the issuance of a loss-of-continuity signal, periodically determiningthe resistance of said conductive wrist strap, producing (a) a saferesistance signal when the determined resistance of said wrist strap isin a predetermined resistance range, (b) a loss-of-resistance signalwhen the determined resistance of said wrist strap is below saidpredetermined resistance range, or (c) a gain-of-resistance signal whenthe determined resistance of said wrist strap is above saidpredetermined resistance range, periodically measuring the surfaceconductivity of said conductive surfaces, generating (a) an adequateconductivity signal when the surface resistance of said surfaces is in apredetermined resistance range, (b) an unsafe resistance signal when thesurface resistance of said surfaces is below said predeterminedresistance range, or (c) an inadequate conductivity signal when themeasured surface resistance of said surfaces is above said predeterminedresistance range, activating a second indicator in response to theproduction of said safe resistance and loss-of-resistance signals and inresponse to the generation of said adequate conductivity and unsaferesistance signals, periodically detecting induced AC voltage on the tipof said soldering iron, producing an unsafe voltage signal when saiddetected AC voltage exceeds a preset voltage, activating a thirdindicator in response to the production of said unsafe voltage signal,and activating an alarm in response to (a) the issuance of saidloss-of-continuity signal, (b) the production of said loss-of-resistancesignal, (c) the generation of said unsafe resistance signal, or (d) theproduction of said unsafe voltage signal.
 19. The method of claim 18further comprising the step of issuing an alarm signal to a centralmonitor in response to the activating of said alarm.
 20. A method formonitoring the operation of static control equipment at a user'sworkstation, said static control equipment including (a) conductivesurfaces such as a conductive workbench mat and a conductive floor mat,(b) a grounded tip soldering iron, and (c) a conductive wrist strap onthe wrist of a user, said conductive surfaces and wrist strap beingconnected to a ground connection, said soldering iron being connected toelectrical AC power having an electrical ground, said method comprisingthe steps of:continuously sensing the low impedance continuity betweensaid ground and said electrical ground, issuing a loss-of-continuitysignal in the event said low impedance exceeds a predeterminedresistance value, periodically determining the resistance of saidconductive wrist strap, producing (a) a safe resistance signal when thedetermined resistance of said wrist strap is in a predeterminedresistance range, (b) a loss-of-resistance signal when the determinedresistance of said wrist strap is below said predetermined resistancerange, or (c) a gain-of-resistance signal when the determined resistanceof said wrist strap is above said predetermined resistance range,periodically measuring the surface conductivity of said conductivesurfaces, generating (a) an adequate conductivity signal when thesurface resistance of said surfaces is in a predetermined resistancerange, (b) an unsafe resistance signal when the surface resistance ofsaid surfaces is below said predetermined resistance range, or (c) aninadequate conductivity signal when the measured surface resistance ofsaid surfaces is above said predetermined conductivity range,periodically detecting induced AC voltage on the tip of said solderingiron, and producing an unsafe voltage signal when said detected ACvoltage exceeds a preset voltage.
 21. A method for monitoring theoperation of static control equipment at a user's workstation, saidstatic control equipment being connected to a ground connection, saidmethod comprising the steps of:continuously sensing the low impedancecontinuity between said earth ground and the electrical ground found atthe user's workstation, issuing a loss-of-continuity signal in the eventsaid low impedance exceeds a predetermined resistance value,periodically determining the resistance of said static controlequipment, and producing (a) a safe resistance signal when thedetermined resistance of said equipment is in a predetermined resistancerange, (b) a loss-of-resistance signal when the determined resistance ofsaid equipment is below said predetermined resistance range, or (c) again-of-resistance signal when the determined resistance of saidequipment is above said predetermined resistance range.
 22. The methodof claim 21 further comprising the steps of:measuring the surfaceconductivity of said static control equipment, and generating (a) anadequate conductivity signal when the surface resistance of saidequipment is in a predetermined resistance range, (b) an unsaferesistance signal when the surface resistance of said equipment is belowsaid predetermined resistance range, or (c) an inadequate conductivitysignal when the surface resistance of said equipment is above saidpredetermined resistance range.
 23. A method for monitoring theoperation of the grounded tip soldering iron at a user's workstation,said soldering iron being connected to electrical AC power having anelectrical ground, said method comprising the steps of:detecting inducedAC voltage on the tip of said soldering iron by periodically touchingthe tip of the soldering iron to a high impedance load, producing anunsafe voltage signal when said detected AC voltage across the highimpedance load exceeds a preset voltage, and activating an indicator inresponse to the production of said unsafe voltage signal.
 24. The methodof claim 23 further comprising the steps of:continuously sensing the lowimpedance continuity between said ground and said electrical ground, andissuing a loss-of-continuity signal in the event said low impedanceexceeds a predetermined resistance value.